So far, it's been a good millennium for British theatre. The West End is healthy, and both the National and the Royal Shakespeare Company have spent most of the decade triumphantly on song. The National Theatre of Scotland pioneered a highly successful model of working without the burdens of a building, a model now taken up in Wales. In 2003, the government injected £25m into the English regional theatre, saving it from precipitous decline.

But the biggest change is a dramatic expansion in a form of theatre that lots of people thought was on the way out: the individually written new play.

For 10 years, academics, funders and commentators have argued that text-based drama is in irreversible decline. Traditionally, the big division in theatres' programmes was between old and new plays. Now a new fault line has been drawn, between a dusty, out-of-date canon of text-based drama – everything from Sophocles to Simon Stephens, from The Bacchae to Blasted – and a vibrant, up-to-the-minute, physically inspired theatre devised by actors. Hence, in the Arts Council's 2007 theatre policy review, new work was dropped as a priority, in favour of "experimental practice and interdisciplinary practice, circus and street arts".

This year the Arts Council has produced several reports on the effects of the 2003 financial uplift. I was one of a group of playwrights and academics commissioned to find out what happened to new writing in the mainstream sector (that's the big national companies, the regional theatres, and subsidised touring companies).

The first thing we found was how little anyone knew about what kind of plays are done in the English theatre. From the mid-80s to the late 90s, theatres told the Arts Council what kind of plays they'd presented (Shakespeare, classics, children's, new work, etc) and how well they did. Roughly, new plays represented 12-20% of the repertoire of building-based companies, concentrated in small studio spaces.

In this decade, the Arts Council stopped asking detailed questions about programming. Theatres are required to report the amount of new work, new commissions and the established repertoire in their programmes, but the definitions are vague (it is not clear if a "new commission" is a newly commissioned play, or could apply to a newly commissioned production). Accordingly, we decided to ask 89 regularly funded English theatre companies what plays they had done (and how well they had done) since 2003; 65 companies responded, and the results were extraordinary.

Since 2003, the amount of new work in the repertoire of the replying companies has more than doubled, making up 42% of all productions. Half the new plays are presented by 10 theatres (including the National, the Royal Court, the RSC and major regional theatres in Birmingham, Liverpool, Manchester and Leeds), but only one of the responding theatres did no new work at all.

One reason for the upsurge is that writers are doing different kinds of work: there have been significant increases in new adaptations and writing for children (20% of all new writing). There has indeed been an increase in work devised by actors (7% of performances), but clearly this form of work is not taking over from individually written new plays. And new plays sold well: over the decade attendances grew, and new work actually did better than the average in the final year of our survey.

But the most striking finding is that new plays have broken out of the studio ghetto. The majority of new plays are now watched in auditoriums with more than 200 seats. Nine out of 10 individual attendances for new plays in our responding theatres were in main houses. And the average box office performance of new plays on main stages was a healthy 65%, and rising.

From John Osborne onwards, new writing has been seen as a jewel of the British theatre, but also as an essentially minority taste. The last decade represents a triumph for Arts Council policy, and for artistic directors who refused to accept the presumption that new plays empty theatres.

There is, however, the obvious paradox that news of this triumph comes exactly at the point when fashionable opinion has turned its back on text-based theatre. It was odd enough, in the mid-90s, for anti-text commentators to dismiss a new-play repertoire dominated by path-breaking playwrights like Sarah Kane and Mark Ravenhill. It's even more perverse to write off new writing now that a generation of young playwrights (including women such as Lucy Prebble, Polly Stenham and Alia Bano) has burst on the scene.

Of course it isn't either/or: text-based and non-text-based theatre are informing each other's practice, as Nick Hytner of the National Theatre argues and exemplifies. But, for 10 years, much public policy thinking, academic study and critical taste was based on the assumption that writing plays was a dying art – while, in fact, there's more of it than ever before.